This invention relates in general to xerography and more specifically to a method of making a photosensitive device.
In the art of xerography, a xerographic plate containing a photoconductive insulating layer is first uniformly electrostatically charged in the dark in order to sensitize the surface of the photoconductive layer. The plate is then exposed to an image of activating electromagnetic radiation, such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind the latent electrostatic image in the nonilluminated areas. The latent electrostatic image may be developed and made visible by depositing finely divided electroscopic marking particles on the surface of the photoconductive layer. This concept was originally described by Carlson in U.S. Pat. No. 2,297,691 and is further amplified and described by many related patents in the field.
Conventional xerographic plates or drums usually comprise a photoconductive insulating layer overlaying a conductive support. A photoconductive material which has had wide use as a reusable photoconductor in commercial xerography comprises vitreous or amorphous selenium. Vitreous selenium in essence comprises super cooled selenium liquid and may readily be formed by vacuum evaporation by cooling the liquid or vapor so suddenly that crystals of selenium do not have time to form. Although vitreous selenium has had wide acceptance for commercial use in xerography, its spectral response is limited largely to the blue-green portion of the electromagnetic spectrum (below about 5200 Angstrom Units). In general, one requirement of a photoconductor, such as vitreous selenium, is that its resistivity should drop at least several orders of magnitude in the presence of activating radiation or light. Also, the photoconductive layer should be able to support a significant electrical potential in the absence of activating radiation.
Selenium also exists in a crystalline form known as trigonal or hexagonal selenium which is well known to the semiconductor art for use in the manufacture of selenium rectifiers. In the crystalline trigonal form, the structure of the selenium consists of helical chains of selenium atoms which are parallel to each other along the crystallographic c-axis. Trigonal selenium is not normally used in xerography as a homogeneous photoconductive layer because of its relatively high electrical conductivity in the dark, although in some instances trigonal selenium can be used in binder structures wherein trigonal selenium particles are dispersed in a matrix of another material such as an electrically active organic material, or a photoconductor such as vitreous selenium.
U.S. Pat. Nos. 2,739,079 and 3,692,521 both describe photosensitive members utilizing small amounts of crystalline hexagonal (trigonal) selenium contained in predominantly vitreous selenium matrices. In addition, copending U.S. patent application Ser. No. 669,915, filed Sept. 22, 1967, describes a special form of red-hexagonal selenium suitable for use in binder structure in which finely divided red-hexagonal selenium particles are contained in a resin binder matrix.
Although trigonal selenium exhibits a wider spectral response than vitreous selenium, as stated above, trigonal selenium is not normally used in xerography because of its relatively high electrical conductivity in the dark. However, imaging structures which are able to use a homogeneous layer of hexagonal selenium would have advantages over those using vitreous selenium with regard to improved spectral response and increased sensitivity. Further, the use of a trigonal selenium layer in a specially constructed xerographic member could provide better overall electrical characteristics than vitreous selenium photoreceptors.